Question

The seventh term of a GP is 4 times the 5th term. What will be the first term when its 4th term is 40? 

(a) 4 

(b) 5 

(c) 3 

(d) 2

Answer 1

Correct option (b) 5

Explanation:

In the case of a G.P. the 7th term is derived by multiplying the 5th term twice by the common ratio. 

(Note: this is very similar to what we had seen in the case of an A.P.) Since, the seventh term is derived by multiplying the 5th term by 4, the relationship. 

r² = 4 must be true. 

Hence, r = 2 

If the 4th term is 40, the series in reverse from the 4th to the first term will look like: 40, 20, 10, 5. Hence,